Communication systems that are subject to space and weight limitations, such as mobile, manually deployable configurations, often employ (monofilar or bifilar) axial mode, helical antennas, such as that diagrammatically illustrated at 10 in FIG. 1. By axial mode helical antenna is meant one that is not only physically configured as a helix, but, as an axial mode device for circularly polarized RF signals, has the principal lobe of its radiation pattern extending along the axis or boresight 12 of the helix, as diagrammatically illustrated at 20 in FIG. 2. Axial length is measured along axis 12.
Moreover, the wavelength of such an axial mode, helical antenna is less than the axial dimension of the antenna. For example, the axial dimension of a helix having a pitch angle on the order of nine degrees and having four to five turns is slightly less than a wavelength; a five to twenty-three degree pitch angle, five turn helix has an axial dimension of 1.2 wavelengths. This is in contrast to a helical-configured monopolar or bipolar antenna, such as a whip antenna, which is formed of a helically wound conductor, but does not operate as an axial mode device, and has a wavelength typically larger than the axial length of the antenna, and much larger than the circumference, which is typically on the order of one-one hundredth of a wavelength.
Because the pitch angle of a conventional axial mode, helical antenna is constant along the axis of the antenna (typically on the order of twelve degrees or so), then at any point along the axis of the antenna, the phase velocity of the electromagnetic wave travelling through the antenna will not necessarily match the phase velocity of the free space wave being interfaced with (received or launched by) the antenna. For the case of a received wave, for example, this phase velocity mismatch prevents the incoming free space wave from coherently exciting currents within the antenna. As a result, the gain of the antenna is reduced to value that is less than optimal.
For a non-limiting examples of such conventional helical antenna configurations, including both axial mode devices, and non-axial mode configurations, such as, but not limited to, whip antennas, attention may be directed to the following documentation: U.S. Pat. No. 3,568,205 to Buxton, U.S. Pat. No. 3,858,220 to Arnow, U.S. Pat. No. 4,087,820 to Henderson, U.S. Pat. No. 4,148,030 to Foldes, U.S. Pat. No. 4,161,737 to Albright, U.S. Pat. No. 4,163,981 to Wilson, U.S. Pat. No. 4,169,267 to Wong et al, U.S. Pat. No. 4,780,727 to Seal et al, U.S. Pat. No. 5,081,469 to Bones, U.S. Pat. No. 5,406,693 to Egashira et al, U.S. Pat. No. 5,479,182 to Sydor, U.S. Pat. No. 5,489,916 to Waterman et al, Japanese Publication No. 7-202550 to Oomuro et al, Japanese Publication No. 7-176940 to Oomuro et al, Japanese Publication No. 7-22839 to Tsutsumi, Japanese Publication No. 7-22830 to Yamamoto.